Field of the Invention
The present invention relates to an ionic liquid, a lubricant containing the ionic liquid, and a magnetic recording medium using the lubricant.
Description of the Related Art
Conventionally, in a thin film magnetic recording medium, a lubricant is applied onto a surface of a magnetic layer for the purpose of reducing frictions between a magnetic head and the surface of the magnetic recording medium, or reducing abrasion. In order to avoid adhesion, such as sticktion, an actual film thickness of the lubricant is of a molecular order. Accordingly, it is not exaggeration to say that the most important thing for a thin film magnetic recording medium is to select a lubricant having excellent abrasion resistance in any environment.
During a life of a magnetic recording medium, it is important that a lubricant is present on a surface of the medium without causing desorption, spin-off, and chemical deteriorations. Making the lubricant present on a surface of a medium is more difficult, as the surface of the thin film magnetic recording medium is smoother. This is because the thin film magnetic recording medium does not have an ability of replenishing a lubricant as with a coating-type magnetic recording medium.
In the case where an adhesion force between a lubricant and a protective film disposed at a surface of a magnetic layer is weak, moreover, a film thickness of the lubricant is reduced during heating or sliding hence accelerating abrasion. Therefore, a large amount of the lubricant is required. The large amount of the lubricant is the mobile lubricant, and therefore a function of replenishing the lost lubricant can be provided. However, an excessive amount of the lubricant makes the film thickness of the lubricant larger than the surface roughness. Therefore, a problem associated with adhesion arises, and in a crucial case, sticktion arises to cause driving failures.
As illustrated in FIG. 1, although an increase rate of an areal density of a hard disk drive of a product have been reduced in the last few years, the increase rate has reached an annual rate of 25%, and has nearly reaches 4 Tb/in2, which is one of targets, in Advances in Tribology Volume 2013, Article ID 521086. As illustrated in FIG. 2, it has been understood that a distance between head disk interfaces relative to an increase in the recording density reduces, but there is a need to always improve reliability corresponding to the reduction in the distance, which can be described, for example, in non-patent literatures below. (C. M. Mate, Q. Dai, R. N. Payne, B. E. Knigge, and P. Baumgart, “Will the numbers add up for sub-7-nm magnetic spacings? Future metrology issues for disk drive lubricants, overcoats, and topographies,” IEEE Transactions on Magnetics, vol. 41, no. 2, pp. 626-631, 2005., B. Marchon and T. Olson, “Magnetic spacing trends: from LMR to PMR and beyond,” IEEE Transactions on Magnetics, vol. 45, no. 10, pp. 3608-3611, 2009., J. Gui, “Tribology challenges for head-disk interface toward 1 Tb/in2,” IEEE Transactions on Magnetics, vol. 39, no. 2, pp. 716-721, 2003.).
A current recording density is about 1 Tb/in2, spacing is about 6 nm, and a thickness of a lubricant is 0.8 nm. The thickness of the lubricant needs to be reduced at the prospective recording density of 4 Tb/in2. In order to reduce a thickness of a film of a PFPE lubricant commonly used in the art, however, a molecular weight of the PFPE lubricant needs to be decreased. The smaller molecular weight of the PFPE lubricant has a problem that thermal stability is deteriorated. It has been understood that the above-described problems associated with reliability have not be sufficiently solved with common perfluoropolyether (PFPE)-based lubricants.
Particularly for a thin film magnetic recording medium having high surface smoothness, a novel lubricant is designed at a molecular level, and synthesized to solve the above-described trade-off. Moreover, there are a number of reports regarding lubricity of PFPE. As described, lubricants are very important in magnetic recording media.
Chemical structures of typical PFPE-based lubricants are depicted in Table 1.
TABLE 1Fomblin-based lubricantsX—CF2(OCF2CF2)n(OCF2)mOCF2—X(0.5 < n/m < 1)ZX = —OCF3Z-DOLX = —CH2OHZ-DIACX = —COOHZ-TetraolX = —CH2OCH2CHCH2OHOH AM2001 Other lubricantsA2OH MonoF—(CF2CF2CF2O)1—CF2CF2CH2—N(C3H7)2
Z-DOL in Table 1 is one of lubricants typically used for thin-film magnetic recording media. Moreover, Z-tetraol (ZTMD) is a lubricant, in which a functional hydroxyl group is further introduced into a main chain of PFPE, and it has been reported that use of Z-tetraol enhances reliability of a drive while reducing a space at an interface between a head and a medium. It has been reported that A20H suppresses decomposition of the PFPE main chain with Lewis acid or Lewis base, and improves tribological properties. On the other hand, it has been reported that Mono has a different polymer main chain and different polar groups to those of the PFPE, and the polymer main chain and polar groups of Mono are respectively poly-n-propyloxy, and amine, and Mono reduces adhesion interactions at near contact.
However, a typical solid lubricant, which has a high melting point and is considered thermally stable, disturbs an electromagnetic conversion process that is extremely highly sensitive, and moreover, an abrasion powder scraped by a head is generated on a running track. Therefore, abrasion properties are deteriorated. As described above, the liquid lubricant has mobility that enables to move the adjacent lubricant layer to replenish the lubricant removed due to abrasion by the head. However, the lubricant is span-off from a surface of the disk especially at a high temperature during driving of the disk, because of the mobility of the lubricant, and thus the lubricant is reduced. As a result, a protection function is lost. Accordingly, a lubricant having a high viscosity and low volatility is suitably used, and use of such a lubricant enables to prolong a service life of a disk drive with suppressing an evaporation rate.
Considering the above-described lubricating systems, requirements for a low-friction and low-abrasion lubricant used for thin film magnetic recording media are as follows.
(1) Low volatility.
(2) Low surface tension for a surface filling function.
(3) Interaction between terminal polar groups and a surface of a disk.
(4) High thermal and oxidization stability in order to avoid decomposition or reduction over a service period.
(5) Chemically inactive with metals, glass, and polymers, and no abrasion powder generated from a head or a guide.
(6) No toxicity and no flammability.
(7) Excellent boundary lubricating properties.
(8) Soluble with organic solvents.
Recently, an ionic liquid has been attracted attentions as one of solvents for synthesis of organic or inorganic materials and being friendly to the environments in the fields of electricity storage materials, a separation technology, and a catalyst technology. The ionic liquid is roughly classified as a molten salt having a low melting point. The ionic liquid is typically a molten salt having a melting point of 100° C. or lower, among the above-mentioned molten salts. The important properties of the ionic liquid used as a lubricant are low volatility, inflammability, thermal stability, and an excellent dissolving performance.
For example, abrasion and wear of a surface of a metal or ceramic may be reduced by using a certain ionic liquid compared to a conventional hydrocarbon-based lubricant. For example, there is a report that, in the case where an imidazole cation-based ionic liquid is synthesized by substituting with a fluoroalkyl group, and tetrafluoroboric acid salt or hexafluorophosphoric acid salt of alkyl imidazolium is used for steel, aluminium, copper, single crystal SiO2, silicon, or sialon ceramics (Si—Al—O—N), tribological properties more excellent than those of cyclic phosphazene (X-1P) or PFPE are exhibited. Moreover, there is a report that an ammonium-based ionic liquid reduces frictions more than a base oil in the region of elastohydrodynamic to boundary lubrication. Moreover, effects of the ionic liquid as an additive for a base oil have been studied, and a chemical or tribochemical reaction of the ionic liquid has been researched to understand lubricating systems. However, there are almost no application examples of the ionic liquid to magnetic recording media.
Among the ionic liquids, a perfluorooctanoic acid alkyl ammonium salt is a protic ionic liquid (PIL), and has been reported as having a significant effect of reducing frictions of magnetic recording media compared to Z-DOL mentioned above (see, for example, Japanese Patent Nos. 2581090 and 2629725, Kondo, H., Seto, J., Haga. S., Ozawa, K. (1989) Novel Lubricants for Magnetic Thin Film Media, Magnetic Soc. Japan, Vol. 13, Suppl. No. 51, pp. 213-218, Kondo, H., Seki, A., Watanabe, H., & Seto, J., (1990). Frictional Properties of Novel Lubricants for Magnetic Thin Film Media, IEEE Trans. Magn. Vol. 26, No. 5, (September 1990), pp. 2691-2693, ISSN: 0018-9464, Kondo, H., Seki, A., & Kita, A., (1994a). Comparison of an Amide and Amine Salt as Friction Modifiers for a Magnetic Thin Film Medium. Tribology Trans. Vol. 37, No. 1, (January 1994), pp. 99-104, ISSN: 0569-8197).
However, the above-mentioned perfluorocarboxylic acid ammonium salts have weak interaction between a cation and an anion in the reaction represented by the following reaction formula (A). According to Le Chatelier's principle, the equilibrium of the reaction is sifted to the left side at a high temperature, and the perfluorocarboxylic acid ammonium salt becomes a dissociated neutral compound and hence thermal stability is deteriorated. Specifically, protons are transferred at a high temperature, the equilibrium is sifted to neutral substance and dissociation occurs (see, for example, Yoshizawa, M., Xu, W., Angell, C. A., Ionic Liquids by Proton Transfer: Vapor pressure, Conductivity, and the Relevance of ΔpKa from Aqueous Solutions, J. Am. Chem. Soc., Vol. 125, pp. 15411-15419 (2003)).CnF2n+1COOH+CnF2n+1NH2⇄CnF2n+1COO−H3N+CnH2n+1  (A)
Meanwhile, the limit of a surface recording density of a hard disk is said to be from 1 Tb/in2 to 2.5 Tb/in2. Currently, the surface recording density is getting close to the limit, but developments of technology for increasing capacities have been actively conducted with a reduction in particle size of magnetic particles as a premise. As the technology for increasing capacities, there are a reduction in an effective flying height and introduction of Shingle Write (BMP).
Moreover, there is “thermally-assisted magnetic recording (heat assisted magnetic recording)” as the next-generation recording technology. The outline of the thermally-assisted magnetic recording is illustrated in FIG. 3. In FIG. 3, the referential numeral 1 is laser light, the referential numeral 2 is near field light, the referential numeral 3 is a recording head (PMR element), and the referential numeral 4 is a reproducing head (TMR element). The problems of the thermally-assisted magnetic recording include a deterioration of durability due to evaporation or deterioration of a lubricant on a surface of a magnetic layer because a recording area is heated by laser during recording and reproducing. Even though it is a short period, there is a possibility that a thin film magnetic recording medium is exposed to a high temperature, which is 400° C. or higher, in thermally-assisted magnetic recording. Therefore, thermal stability of a lubricant generally used for thin film magnetic recording media, such as Z-DOL and Z-TETRAOL, is considered.
A protic ionic liquid forms ions as described above, and is typically a material having high thermal stability. The equilibrium of the protic ionic liquid is represented by Scheme 1 below.
      Scheme    ⁢                  ⁢    1    ⁢                  ⁢    Scheme    ⁢                  ⁢    of    ⁢                  ⁢    acid    ⁢          -        ⁢    base    ⁢                  ⁢    reaction              HA      +                        H          2                ⁢        O              ⇄                            H          3                ⁢                  O          +                    +              A        -                        B      +                        H          2                ⁢        O              ⇄                  HB        +            +              OH        -                                A        -            +              HB        +              →                  A        -            ⁢              HB        +                                HA        +        B        +                  2          ⁢                      H            2                    ⁢          O                    ⇄                                    A            -                    ⁢                      HB            +                          +                              H            3                    ⁢                      O            +                          +                  OH          -                      _  
In Scheme 1, HA is Bronsted acid and B is Bronsted base. The acid (HA) and the base (B) are reacted as in Scheme 1 to generate a salt (A−HB+).
In Scheme 1, a dissociation constant Ka1 of the acid and a dissociation constant Kb2 of the base can be represented by Scheme 2 below in the form including the density.
      Scheme    ⁢                  ⁢    2    ⁢                  ⁢    Relationship    ⁢                  ⁢    of    ⁢                  ⁢    acid    ⁢                  ⁢    and    ⁢                  ⁢    base    ⁢                  ⁢    dissociation    ⁢                  ⁢    constants              K              a        ⁢                                  ⁢        1              =                            [                      A            -                    ]                ⁡                  [                                    H              3                        ⁢                          O              +                                ]                            [        HA        ]                        K              b        ⁢                                  ⁢        2              =                            [                      HB            +                    ]                ⁡                  [                      OH            -                    ]                            [        B        ]            
The Ka1 and Kb2 largely differ depending on a substance. In some cases, the Ka1 and Kb2 may be large digits, which is inconvenient for handling. Therefore, it is often represented with a negative logarithm. Specifically, the acid dissociation constant is determined as −log10Ka1=pKa1 as represented by Scheme 3 below. Clearly, acidity is stronger, as the pKa1 is smaller.
A difference ΔpKa of the acid dissociation constants of the acid and the base is discussed. The acid-base reaction is influenced by both acidity of the acid and basicity of the base (or acidity of conjugate acid of the base), and the difference ΔpKa in the acidity of the acid and base is represented by Scheme 3 below.
      Scheme    ⁢                  ⁢    3    ⁢                  ⁢    Relationship    ⁢                  ⁢    of    ⁢                  ⁢    pKa    ⁢                  ⁢    of    ⁢                  ⁢    acid    ⁢                  ⁢    and    ⁢                  ⁢    base              pK              a        ⁢                                  ⁢        1              =                  -        log            ⁢                                    [                          A              -                        ]                    ⁡                      [                                          H                3                            ⁢                              O                +                                      ]                                    [          HA          ]                                pK              b        ⁢                                  ⁢        2              =                  -        log            ⁢                                    [                          HB              +                        ]                    ⁡                      [                          OH              -                        ]                                    [          B          ]                                pK              a        ⁢                                  ⁢        2              =                  14        -                  pK                      b            ⁢                                                  ⁢            2                              =              14        +                  log          ⁢                                                    [                                  BH                  +                                ]                            ⁡                              [                                  OH                  -                                ]                                                    [              B              ]                                                                                    Δ            ⁢                                                  ⁢                          pK              a                                =                                                    pK                                  a                  ⁢                                                                          ⁢                  2                                            -                              pK                                  a                  ⁢                                                                          ⁢                  1                                                      =                                          -                                  pK                                      a                    ⁢                                                                                  ⁢                    1                                                              -                              pK                                  b                  ⁢                                                                          ⁢                  2                                            +              14                                                                    =                      log            ⁢                                                                                                      [                                              A                        -                                            ]                                        ⁡                                          [                                              BH                        +                                            ]                                                        ⁡                                      [                                                                  H                        3                                            ⁢                                              O                        +                                                              ]                                                  ⁡                                  [                                      OH                    -                                    ]                                                                              [                  HA                  ]                                ⁡                                  [                  B                  ]                                                                                                  =                                    log              ⁢                                                                    [                                          A                      -                                        ]                                    ⁡                                      [                                          HB                      +                                        ]                                                                                        [                    HA                    ]                                    .                                      [                    B                    ]                                                                        =                          log              ⁢                                                [                                                            A                      -                                        ⁢                                          HB                      +                                                        ]                                                                      [                    HA                    ]                                    ⁡                                      [                    B                    ]                                                                                          
It is indicated that the ΔpKa increases, as the base concentration [A−HB+] increases relative to the acid concentration and the base concentration.
Meanwhile, Yoshizawa et al. have reported that proton transfer tends to occur when a difference (ΔpKa) of pKa of acid and base is 10 or greater,[AH]+[B]↔[A−HB+]the equilibrium of the formula above is sifted to the ion side (right side), and stability is enhanced further (see, for example, Yoshizawa, M., Xu, W., Angell, C. A., Ionic Liquids by Proton Transfer: Vapor pressure, Conductivity, and the Relevance of ΔpKa from Aqueous Solutions, J. Am. Chem. Soc., Vol. 125, pp. 15411-15419 (2003)). Moreover, Watanabe et al. has reported that proton transfer and thermal stability of a protic ionic liquid largely depend on ΔpKa, and thermal stability of the ionic liquid is significantly improved by using the acid with which ΔpKa is 15 or greater when DBU (1,8-diazabicyclo[5,4,0]undec-7-ene) is used as a base (see, for example, Miran, M. S., Kinoshita, H., Yasuda, T., Susan, M. A. B. H., Watanabe, M., Physicochemical Properties Determined by ΔpKa for Protic Ionic Liquids Based on an Organic Super-strong Base with Various Bronsted Acids, Phys. Chem. Chem. Phys., Vol. 14, pp. 5178-5186 (2012)). Moreover, Kondo et al. have reported that a perfluorooctanesulfonic acid octadecyl ammonium salt-based protic ionic liquid having large ΔpKa improves durability of a magnetic recording medium (see, for example, Hirofumi Kondo, Makiya Ito, Koki Hatsuda, Kyung Sung Yun and Masayoshi Watanabe, “Novel Ionic Lubricants for Magnetic Thin Film” Media, IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, pp. 3756-3759, July (2013), WO 2014/104342). In the recent report of Kondo et al. related to thermal resistance of an ionic liquid, it has been reported that a decomposition temperature increases with up to certain degree of ΔpKa, and the decomposition temperature is not increased any further even when ΔpKa is increased from the above-described point (see, for example, Hirofumi Kondo, Makiya Ito, Koki Hatsuda, Nobuo Tano, KyungSung Yun and Masayoshi Watanabe, IEEE International magnetic conference Dresden, Germany, May 4-8, 2014, and Hirofumi Kondo, Makiya Ito, Koki Hatsuda, Nobuo Tano, Kyung Sung Yun and Masayoshi Watanabe, IEEE Trans. Magn., 2014, Vol. 50, Issue 11, Article#: 3302504). Moreover, it is reported that a pyrrolidinium-based ionic liquid including germinal dication may improve thermal resistance more than a typical ionic liquid of monocation (see Anderson, J. L., Ding R., Ellern A., Armstrong D. W., “Structure and Properties of High Stability Geminal Dicationic Ionic Liquids”, J. Am. Chem. Soc., 2005, 127, 593-604). As disclosed in Anderson, J. L., Ding R., Ellern A., Armstrong D. W., “Structure and Properties of High Stability Geminal Dicationic Ionic Liquids”, J. Am. Chem. Soc., 2005, 127, 593-604., however, a relationship between a molecular structure constituting the pyrrolidinium-based ionic liquid and physical or chemical characteristics has not been fully understood yet. A combination of a cation and an anion largely influences on physical or chemical characteristics of an ionic liquid. A variety of the anion site is many, but the relationship is not clear unless the cation is a cation structurally similar to the anion (see, for example, Dzyuba, S. V.; Bartsch, R. A., “Influence of Structural Variations in 1-Alkyl(aralkyl)-3-Methylimidazolium Hexafluorophosphates and Bis(trifluoromethylsulfonyl)imides on Physical Properties of the Ionic Liquids, Chem. Phys. Phys. Chem. 2002, 3, 161-166). For example, viscosity of the liquid increases, as hydrogen bonding strength of halogen is stronger (Cl>Br>I). However, the method for increasing the viscosity is not limited to the increase in the hydrogen bonding strength. For example, the viscosity can be increased by varying an alkyl chain of imidazole. Similarly, the combination of the anion and cation influences melting point, surface tension, and thermal stability, but a wide range of researches has not been conducted on an effect of the anion. Accordingly, it is possible to change physical or chemical characteristics of an ionic liquid by with a combination of cations or anions, but it is difficult to predict.
In the case where the lubricants for hard disks presented in Table 1 are considered, a polar group, such as a hydroxyl group, is introduced at a terminal in order to enhance interaction with a surface of a medium. Such a hydroxyl group reacts with the surface of the medium to be fixed on the surface as a result of a heat treatment. As a result, thermal stability is improved. Moreover, there is also an effect of reducing polar site components of surface energy because the hydroxyl group is bonded (see R. J. Waltman, D. J. Pocker, G. W. Tyndall, Studies on the interactions between ZDOL perfluoropolyether lubricant and the carbon overcoat of rigid magnetic media, Tribology Letters 1998, Volume 4, Issue 3, pp. 267-275).
Meanwhile, long-chain fatty acids or esters of long-chain fatty acids have been used as lubricants known in the art. In the case where a lubricant is used in combination with an ionic liquid, the ionic liquid needs to have excellent solubility to a hydrocarbon-based solvent used for the lubricant.